In the RF transmission of digital information, digital data is mapped to symbols to be modulated onto a quadrature carrier. The quadrature modulator can be applied in the digital domain, producing a real IF digital signal, or it can be applied in the analog domain. As a digital quadrature modulator is perfect, the result is a single side band, suppressed carrier analog signal. An analog quadrature modulator (AQM) is not perfect; gain, phase and DC imbalances between the two baseband signal paths cause image and Local Oscillator (LO) frequency components to leak through onto the resultant analog modulated signal.
The modulated analog signal is either converted directly to the desired RF operating frequency, or through multiple conversion stages. The RF signal is then amplified to the desired output power with a power amplifier. Power amplifiers exhibit strong non-linearity, which, for variable amplitude modulation schemes, such as WCDMA, result in considerable spectral leakage into neighboring frequency bands. As this leakage is strictly regulated by government agencies, it must be minimized by either operating the power amplifier in a more linear region, which impacts the transmitter efficiency, or by employing a linearization scheme, such as feed-forward or digital predistortion.
Many systems employing digital predistortion linearization schemes require large bandwidths to accommodate the compensation signal. Existing digital to analog converter (DAC) technology prevents the use of digital quadrature modulators for wide bandwidth systems, as the maximum bandwidth achievable is limited to FDAC/2 Hz. An analog baseband system utilizing an analog quadrature modulator, however, has effectively double the bandwidth capability.
In multi-carrier transmission systems, the complex baseband signal can have multiple frequency-offset carriers. If these carriers are symmetrical about 0 Hz, the images created from imperfections in the analog quadrature modulator are hidden under the opposite carrier. However, to aid the design of the transmitter, many systems are able to position the baseband carriers at arbitrary offset frequencies. In this situation the images are visible and can, if not minimized, break the spectral emission requirements.
The quality of the image side band suppression is dependent on the quality of the gain and phase balance between the in-phase and quadrature paths, manifested either on the input signal paths of the modulator or in the internal local oscillator. Additionally, any relative DC offset between the in-phase and quadrature paths degrades the carrier suppression.
Accordingly a need presently exists for a system and method for compensation for gain and phase imbalances which is applied to the in-phase and quadrature paths prior to the analog quadrature modulator of a communications system.